Trailing badly in the polls and facing the prospect of a humiliating defeat in his home state of Pennsylvania, Mr Santorum used Gettysburg, the site of the bloodiest battle of the American Civil War, to announce his withdrawal.

"We made the decision to get into this race at our kitchen table against all the odds and we made a decision over the weekend that while this presidential race is over for me - and we will suspend our campaign effective today - we're not done fighting," he said.

While he used his brief appearance to attack President Barack Obama, the one-time candidate did not offer any endorsement of Mr Romney, who now faces an essentially open road to the Republican nomination.

The former Pennsylvania senator said he had "tried to be a witness" to the struggles of America's middle class and a voice for social conservatives.

"Even when they said we couldn't win, we were winning because we were touching hearts and raising issues that frankly a lot of people didn't want raised," he said.

His decision came just hours after he returned to the campaign trail following the hospitalisation of his three-year-old daughter Bella. He took a four-day break from the campaign to care for the child, who has a life-threatening genetic disorder known as Trisomy 18.

He said the time at his daughter's bedside had been used for "prayer and thought" about the future of his candidacy.

Mr Santorum entered the race with virtually no money and fought a shoe string campaign in Iowa, often travelling with only a single aide in a pickup truck that he drove himself.

After months at the very bottom of the polls, the conservative surged in the final days before the January caucuses off the back of support from Christian evangelicals.

On the night of the contest Mr Romney was declared the winner by eight votes but after a recount the result was overturned and handed to Mr Santorum, who won by only 34.

He struggled in New Hampshire and was all but eclipsed by Newt Gingrich after the former Speaker of the House won the South Carolina primary and threatened to defeat Mr Romney in Florida.

But after several weeks in the shadows he roared back into the race with victories in Colorado, Minnesota and Missouri and looked likely to take Mr Romney's home state of Michigan.

However, defeat there and in a string of major states including Illinois and Iowa once again relegated him to the second tier as Mr Romney's financial advantages and superior organisation opened up an insurmountable delegate lead.

He ends his candidacy having claimed 11 states, mainly in the South and Midwest, a record of victory that few would have thought possible.

Mr Gingrich and Ron Paul, the libertarian Texas congressman, remain in the race but neither have the resources to pose any real threat to the Romney camp.

Many conservatives hope that Mr Romney will pick Mr Santorum as his running mate, in an attempt to rally the Republican base around his candidacy, although the prospect is thought to be unlikely.

The two men fought bitterly throughout the campaign, with the the former senator at one point calling Mr Romney "the worst Republican in the country to put up against Barack Obama" on account of his Massachusetts healthcare reforms, which were the basis for the president's own plans.

Within minutes of Mr Santorum's announcement, the Romney campaign put out a statement commending their former rival.

“Senator Santorum is an able and worthy competitor, and I congratulate him on the campaign he ran. He has proven himself to be an important voice in our party and in the nation," Mr Romney said.

"We both recognise that what is most important is putting the failures of the last three years behind us and setting America back on the path to prosperity.”

A spokesman for Mr Santorum said they were in talks with Romney aides about the possibility of staging an official endorsement but that no decision had been made yet.